The present invention relates to a plasma display panel used for display devices and the like and a method for producing the same, and more specifically to a plasma display panel having a partition wall formed by a thermal spraying technique and a method for producing the same, especially a process for forming a partition wall by the thermal spraying technique.
A plasma display panel, which has recently been a target of attention as being suitable for a thin display device, has, for example, a structure shown in FIG. 1. The plasma display device shown in FIG. 1 includes a front substrate 300 and a rear substrate 301 which are located to face each other. Display electrodes 302 and 303, a dielectric layer 304, and a MgO dielectric protective layer 305 are sequentially formed on the front substrate 300. An address electrode 306 and a dielectric layer 307 are formed on the rear substrate 301, and partition walls 308 are formed thereon. A fluorescent layer 309 is applied to a side surface of each partition wall 308.
Discharge gas 310 (for example, a Ne-Xe mixture gas) is sealed between the front substrate 300 and the rear substrate 301 at a pressure of 500 to 600 Torr. The discharge gas 310 is discharged between the display electrodes 302 and 303 to generate ultraviolet rays, and the fluorescent layer 309 is irradiated with the ultraviolet rays to realize image display including color image display.
The partition walls 308 are provided for forming a microscopic discharge space for each of pixel element colors (G, B, R), thus to form discharge cells. The partition walls 308 allow discharge to be controlled on a cell-by-cell basis. Thus, erroneous discharge and erroneous display can be prevented. The sizes of the partition walls 308 are typically as follows in a 40-inch NTSC panel: the partition wall pitch per color is 360 xcexcm; the width of a top end of the partition wall is 50 xcexcm to 100 xcexcm; and the height of the partition wall is 100 xcexcm to 150 xcexcm.
Conventional methods for forming a partition wall include (1) a printing technique, (2) a sand-blasting technique, (3) a photo-pasting technique, and (4) a photo-burying technique (or a lift-off technique). By the printing technique, the partition wall is formed using a screen printing technology. By the sand-blasting technique, a partition wall material is applied on the entire surface of the rear substrate, then a photosensitive film layer is formed on the partition wall material to form a prescribed pattern by a photography technique. An unnecessary portion of the partition wall material is removed by sand-blasting to remove the photosensitive film layer, thereby forming the partition wall. By the photo-pasting technique, a photosensitive paste is applied, and then an unnecessary portion is removed by a photography technique to form the partition wall. By the photo-burying technique (or a lift-off technique), a photosensitive film layer is formed on a substrate, and then a prescribed pattern is formed by a photography technique. A paste is buried in a groove of the pattern, the photosensitive film layer is removed, and then the paste is solidified by baking.
However, these conventional methods have the following problems.
By the printing technique, a partition wall having a height of only about 10 xcexcm can be formed by one cycle of printing. Thus, the printing cycle and the drying cycle need to be repeated in order to form a partition wall having a height of about 100 xcexcm. Such a repetition increases the number of steps and raises the cost. Moreover, as the screen becomes larger, the non-linear expansion and contraction of the screen plate becomes conspicuous, which results in a larger dispersion in the position, thickness and shape of the partition walls.
The sand-blasting technique has the problems that a large amount of material needs to be removed, and that the amount of the material to be removed is difficult to be controlled and thus the substrate and the electrodes are likely to be damaged. The photo-pasting technique has the problem of the expensive pasting material. The photo-burying technique realizes a high precision plasma display panel but cannot realize low-cost production due to the baking step required for forming the partition wall.
The invention described herein made for overcoming the above-described problems of the conventional art makes possible the advantages of (1) providing a method for producing a plasma display panel realizing high quality display by forming a partition wall by a thermal spraying technique at a high precision at low cost; and (2) providing a plasma display panel which has a partition wall produced at a high precision at low cost and thus can realize high quality display.
According to a plasma display panel of the present invention, a partition wall for defining a discharge space is formed of a thermal-sprayed film formed by thermal spraying of a partition wall material. Thus, the above-described objectives are achieved.
In one embodiment, the plasma display panel includes a pair of substrates; and an electrode, a dieleetric layer and a fluorescent layer located between the pair of substrates. The partition wall is located between the pair of substrates. A gas medium is a sealed in the discharge space, and ultraviolet rays generated by discharge of the gas medium are converted into visible light when the fluorescent layer is irradiated, and thus the plasma display panel is lit.
The partition wall may be formed of a partition wall material of a first color from a bottom thereof to a prescribed height and is formed of a partition wall material of a second color from the prescribed height to a top end of the partition wall. For example, the first color is white and the second color is black. The partition wall material of the first color may be aluminum oxide or spinel and the partition wall material of the second color may be chromium oxide, titanium oxide, or a mixture or a melt of aluminum oxide and titanium oxide.
The thermal spraying may be plasma thermal spraying.
A method for producing a plasma display panel according to the present invention includes a partition wall formation process of forming a partition wall defining a discharge space of a thermal-sprayed film formed by thermal spraying of a partition wall material. Thus, the above-described objectives are achieved.
In one embodiment, the partition wall formation process includes the steps of forming a photosensitive coating layer on a substrate; forming an opening having a prescribed pattern in the photosensitive coating layer; depositing the partition wall material to a prescribed height at least inside the opening by the thermal spraying technique, thereby forming the thermal-sprayed film; removing a portion of the thermal-sprayed film bulging out from a surface of the photosensitive coating layer by polishing; and removing the photosensitive coating layer to obtain the partition wall having a prescribed shape.
The thermal spraying may be plasma thermal spraying.
In one embodiment the opening having a prescribed pattern formed on the photosensitive coating layer has a trapezoidal cross-section in which a bottom end is wider than a top end, and the base angle of the cross-section is 60 degrees or more and less than 90 degrees.
A width of the top end of the opening having the prescribed pattern formed on the photosensitive coating layer may be 100 xcexcm or less.
In one embodiment, the partition wall has a multiple-layer structure including at least a first layer formed by depositing a first partition wall material at a plasma energy quantity A and a second layer formed by depositing a second partition wall material on the first layer to a prescribed height at a plasma energy quantity smaller than the plasma energy quantity A, where A is the plasma energy quantity at the time of thermal spraying in an area where at least thermal spray particles tightly adhere to the substrate.
The thermal spraying may be plasma thermal spraying which is performed using at least argon gas or a mixture gas of argon gas and helium gas as plasma operating gas.
In the case where the plasma operating gas is the mixture gas of argon gas and helium gas, the plasma energy quantity may be changed by changing the amount of helium gas.
Preferably, where the width of the top end of the opening having the prescribed pattern formed on the photosensitive coating layer is Wa, a primary particle diameter D of powder of the partition wall material is in the range of 5 xcexcm or more and 0.7xc2x7Wa or less.
The step of forming the thermal-sprayed film may include the step of removing thermal spray particles adhering to the photosensitive coating layer simultaneously with the deposition of the partition wall material.
The step of forming the thermal-sprayed film may include the stop of heating the substrate from a rear surface thereof to maintain a temperature distribution in the substrate within a prescribed range. Alternatively, the step of forming the thermal-sprayed film may include the step of cooling a surface of the substrate to maintain a temperature distribution in the substrate within a prescribed range. Still alternatively, the step of forming the thermal-sprayed film may include the step of heating the substrate from a rear surface thereof and cooling a surface of the substrate to maintain a temperature distribution in the substrate within a prescribed-range.
Preferably, the step of polishing includes the stop of removing the photosensitive coating layer from the surface thereof to a depth of about 10 xcexcm.
Preferably, the stop of forming the thermal-sprayed film includes the stop of moving a thermal spraying torch in a direction perpendicular to a longitudinal direction of the partition wall.
The method may further include the step of forming a fluorescent layer after the photosensitive coating layer is removed.
In one embodiment, the method further include the steps of filling a nick in the partition wall with a prescribed paste material; and forming a fluorescent layer by baking after the photosensitive coating layer is removed. The prescribed paste material is cured by baking of the fluorescent layer.
In one embodiment, the partition wall is formed by thermal spraying of a partition wall material of a first color from a bottom end thereof to a prescribed height and is formed by thermal spraying of a partition wall material of a second color from the prescribed height to a top end of the partition wall. For example, the first color is white and the second color is black. The partition wall material of the first color may be aluminum oxide or spinal, and the partition wall material of the second color may be chromium oxide, titanium oxide, or a mixture or a melt of aluminum oxide and titanium oxide.
According to the present invention having the above-described features, a groove having a prescribed pattern is formed in a photosensitive coating layer, a partition wall material is then deposited in the grooves by a thermal spraying technique (for example plasma thermal spraying technique), and then the photosensitive coating layer in removed. In this manner, high precision partition wall formation is realized without requiring baking. Thus, the production cost of the partition wall is reduced. In the case where the bottom portion of the partition wall is formed of a white material and the top portion of the partition wall is formed of a black material, the resultant partition wall can provide a high luminance and a high contrast.
More specifically, according to the present invention, a thermal spraying technique (for example, plasma thermal spraying technique) is used to directly deposit a partition wall material on a substrate for forming a partition wall on the substrate. In this manner, baking is eliminated. As a result, a space for an electric oven and power supply required for baking are eliminated, thus reducing the cost.
In the case where the partition wall is formed of a white material from a bottom end to a certain height, the reflectance of the visible light of the partition wall can be improved. Thus, the utilization factor of visible light generated by discharge of the fluorescent portion applied on a side surface of the partition wall is improved; and as a result, the luminance of the plasma display panel is raised. In the case where the partition wall is formed of a black material from the certain height to a top end, the reflection of external light is suppressed. As a result, the contrast of the plasma display panel is increased.
In the case where aluminum oxide is used as the white material, a high reflectance of the visible light is realized. Thus, the luminance of the plasma display panel is improved. In the case where at least either one of chromium oxide and titanium oxide is used as the black material, a low reflectance of the external light is realized. Thus, the contrast of the plasma display panel is enhanced.
In the case where an opening having a prescribed pattern is formed in a photosensitive coating layer provided on the substrate and then a partition wall material is deposited in the opening by a thermal spraying technique (for example, plasma thermal spraying technique) to form a partition wall, the shape of the partition wall is limited to the shape of the pattern of the opening formed in the photosensitive coating layer. Accordingly, highly precise partition wall formation is realized.
In the case where the cross-section of the opening formed in the photosensitive coating layer is trapezoidal with the bottom end being wider than the top end and where the base angle of the trapezoidal cross-section is 60 degrees or more and less than 90 degrees, the partition wall material fills and is deposited in the opening having the prescribed pattern. Thus, the shape precision of the partition wall and the adhesiveness of the partition wall to the substrate can be made uniform. Furthermore, the removal of the photosensitive coating layer can be facilitated to eliminate nicks in the partition wall. When the width of the top end of the opening in the photosensitive coating layer is 100 xcexcm or less, the adhesive force between the partition wall and the substrate is small. Accordingly, the partition wall may possibly be removed together with the photosensitive coating layer by the force in the direction of the partition wall which is generated when the photosensitive coating layer is removed. However, according to the present invention, even when the width of the top end of the partition wall is small, the force pushing the partition wall can be alleviated and thus the photosensitive coating layer can be removed without removing the partition wall.
The partition wall can have a multiple-layer structure including at least a first layer formed by depositing a first partition wall material at a plasma energy quantity A and a second layer formed by depositing a second partition wall material on the first layer to a prescribed height at a plasma energy quantity smaller than the plasma energy quantity A, where A is the plasma energy quantity at the time of thermal spraying in an area where at least thermal spray particles tightly adhere to the substrate. In this case, the adhesive force between the substrate and the thermal spray particles can be secured, and also the oxygen defect of the second partition wall material (for example, aluminum oxide) can be reduced to guarantee the whiteness of the partition wall material.
Argon, or a mixture gas of argon and helium, can be used as the plasma operating gas, and the plasma energy quantity can be changed by changing the mixed amount of helium. In this case, when a smaller amount of helium is mixed for forming the second layer than for forming the first layer, the second layer can be formed at a smaller plasma energy quantity than that required for forming the first layer.
In the case where the primary particle diameter D of the powder of the partition wall material used for forming the partition wall is set to be 5 xcexcm or more and 0.7xc2x7Wa or less where Wa is the top width of the opening, the following effects are provided. When the primary particle diameter D of the powder of the partition wall material is 5 xcexcm or more, the partition wall material can be injected into the plasma jet efficiently. As a result, the utilization factor of the partition wall material for forming the partition wall by the plasma thermal spraying technique can be raised. When the primary particle diameter D of the powder of the partition wall material is 0.7xc2x7Wa or less, the opening in the photosensitive coating layer can be efficiently filled with the partition wall material.
When the thermal spray particles adhere to the photosensitive coating layer and a part of the opening (groove) having the prescribed pattern formed in the photosensitive coating layer is covered with the adhering particles, the thermal spray particles are prevented from being deposited in that part, which causes a nick in the partition wall. In order to avoid this, a removing mechanism having a prescribed shape, for example, a squeegee having a sharp end in contact with the photosensitive coating layer, is moved in association with the thermal spraying torch. Consequently, the thermal spraying particles adhering to the photosensitive coating layer and exerting such an undesirable influence are removed. Thus, the thermal-sprayed film can be deposited uniformly and densely in the opening (groove) having the prescribed pattern in the photosensitive coating layer.